Saturday, March 2, 2013

The Lightning Digital AV Adapter Surprise

Cabel Sasser notes the limited resolution, lag, and MPEG artifacts when using the adapter:

There’s a lot more going on in this adapter than we expected: indeed, we think the Lightning Digital AV Adapter outputs video by using AirPlay (or similar MPEG streaming).

The adapter apparently has an ARM system on a chip with 256 MB of RAM. It doesn’t work right away when you plug it in because you have to wait for it to boot.

Commenter Apple Neednotknow:

From talking to a friend from Apple last thanksgiving, this was done because of the lack of pins, and the iOS device starts by pushing firmware to the dongle then streams video to the dongle which outputs the hdmi signal. The dongle and firmware combo were described to me as basically an underpowered Apple TV minus the wireless hardware.

Commenter Common Sense:

This is pathetic. We thought the lack of analog audio line out was a major blunder, but this is just unbelievably bad. Reasonable people could understand that Apple needed a new dock connector, but that would be to provide BETTER signals, not worse.

Commenter Frank:

The idea is great: The lightning adapter will be Apples only port and it will remain future proof.

In reality however, the lightning connector also has a limited bandwidth thus they have to compress their data, the conversion to and from the Apple lightning protocol requires lots of processing power, on both sides, and reduces quality. And finally it’s expensive.

On Reddit, thisisnotdave writes:

The Lighting Digital AV adapter does in fact do 1080p for video playback! It DOES NOT do it for screen mirroring, which suck, but its important to make that distinction since neither OP nor the article do so.


The lightning connector and cable can all support huge amounts of bandwidth, at least USB 3.0 levels, but the NAND controller in the current batch of iDevices can’t. The connector itself is pretty future-proof, though.

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Anonymous Coward, has a very interesting comment. (En-Lightning?)

Actually I'm just going to repost that here just in case something happens to it. Once again, not from me.

Airplay is not involved in the operation of this adapter.

It is true that the kernel the adapter SoC boots is based off of XNU, but that’s where the similarities between iOS and the adapter firmware end. The firmware environment doesn’t even run launchd. There’s no shell in the image, there’s no utilities (analogous to what we used to call the “BSD Subsystem” in Mac OS X). It boots straight into a daemon designed to accept incoming data from the host device, decode that data stream, and output it through the A/V connectors. There’s a set of kernel modules that handle the low level data transfer and HDMI output, but that’s about it. I wish I could offer more details then this but I’m posting as AC for a damned good reason.

The reason why this adapter exists is because Lightning is simply not capable of streaming a “raw” HDMI signal across the cable. Lightning is a serial bus. There is no clever wire multiplexing involved. Contrary to the opinions presented in this thread, we didn’t do this to screw the customer. We did this to specifically shift the complexity of the “adapter” bit into the adapter itself, leaving the host hardware free of any concerns in regards to what was hanging off the other end of the Lightning cable. If you wanted to produce a Lightning adapter that offered something like a GPIB port (don’t laugh, I know some guys doing exactly this) on the other end, then the only support you need to implement on the iDevice is in software- not hardware. The GPIB adapter contains all the relevant Lightning -> GPIB circuitry.

It’s vastly the same thing with the HDMI adapter. Lightning doesn’t have anything to do with HDMI at all. Again, it’s just a high speed serial interface. Airplay uses a bunch of hardware h264 encoding technology that we’ve already got access to, so what happens here is that we use the same hardware to encode an output stream on the fly and fire it down the Lightning cable straight into the ARM SoC the guys at Panic discovered. Airplay itself (the network protocol) is NOT involved in this process. The encoded data is transferred as packetized data across the Lightning bus, where it is decoded by the ARM SoC and pushed out over HDMI.

This system essentially allows us to output to any device on the planet, irregardless of the endpoint bus (HDMI, DisplayPort, and any future inventions) by simply producing the relevant adapter that plugs into the Lightning port. Since the iOS device doesn’t care about the hardware hanging off the other end, you don’t need a new iPad or iPhone when a new A/V connector hits the market.

Certain people are aware that the quality could be better and others are working on it. For the time being, the quality was deemed to be suitably acceptable. Given the dynamic nature of the system (and the fact that the firmware is stored in RAM rather then ROM), updates **will** be made available as a part of future iOS updates. When this will happen I can’t say for anonymous reasons, but these concerns haven’t gone unnoticed.

@Jesper Thanks. I updated your link because it was pointing to the Apple Neednotknow comment that I linked to, rather than Anonymous Coward.

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